The action of colicin K on the membrane ATPase will be studied. The enzyme in membranes from colicin-treated cells is less sensitive to inhibition by ADP. Reconstitution of the membrane-bound form of the enzyme will reveal whether the enzyme itself or another component of the membrane is altered by colicin. Use of right-side-out and inverted vesicles will show which side of the membrane is attacked by colicin. Colicin covalently attached to sephadex will also show whether entry into the cell is necessary for killing. Wild-type sensitive cells can survive treatment with colicin if they are plated on media containing 100mM K ion and 1.5mM Mg ions. The recovery process may follow desorption or degradation of the colicin, which will be detected using I125-colicin molecules. The optimal Mg ions concentration (1-1.5mM) for survival of colicin-treated cells, which are passively permeable to cations, should be equal to the free internal concentration that is needed for growth. We can select permeable mutants by eliminating those cells able to grow at lower or higher concentrations of Mg ions. Transport in such mutants should be mediated by an uncoupled, facilitated diffusion system or possibly by excessive amounts of a Mg ions ionophore.